1. Field of the Invention
The present invention relates to an optical transmission system and in particular to a performance monitoring technique for large-capacity and long-distance transmission requiring error correction processing.
2. Description of the Related Art
With the recent increase in data transmission capacity, SONET (Synchronous Optical NETwork) or SDH (Synchronous Digital Hierarchy) has been employed as a basic transmission scheme for fiber-optic communication systems.
In the SONET/SDH systems, the parity check bytes such as B1, B2, and B3 in the SONET/SDH frame are used to compute a transmission error rate based on BIP (Bit Interleaved Parity) calculation. The frame format of SONET/SDH is shown in FIG. 6, where B1 byte is used for section (regenerator section for SDH) bit error rate (BER) monitoring, B2 byte for line (multiplex section for SDH) BER monitoring, and B3 byte for path BER monitoring.
A parity check byte is computed from all or a predetermined part of the previous frame for each of section, line and path and is written into a corresponding one of B1, B2 and B3 bytes. Therefore, the transmission BER for each of section, line and path can be computed to allow performance monitoring therefor.
With the vast increase in data transmission capacity, error-correction techniques compensating for transmission errors have been employed in the SONET/SDH systems. In this case, however, the parity check byte cannot be used as it is for performance monitoring after error correction. Hereafter, the details will be described, taking as an example the case of parity check bit for simplicity.
It is assumed that one bit is corrected in an N-bit frame having a parity check bit included in the overhead thereof and an actual transmission error rate is Pe.
In the case where the error correction is not performed, the parity check bit allows one bit error to be detected. Accordingly, an error rate Pe—bip obtained from the parity check bit is calculated by the following expression:Pe—bip=(1/N)·{1−(1−Pe)N}.
According to this expression, if the actual transmission error rate Pe is sufficiently small, then the calculated error rate Pe—bip is approximately equal to the actual transmission error rate Pe.
Contrarily, in the case where the error correction is performed, one bit error is corrected and the number of error bits becomes 0, but k (k>1) bit errors produce (k+1) bit errors due to miscorrection. Therefore, an error rate Pe—fec obtained by performing the error correction is calculated by the following expression:
      P    e_fec    =            (              1        /        N            )        ·                  ∑                  k          =          0                N            ⁢                          ⁢                                    (                          k              +              1                        )                    ⁢                      ·            N                    ⁢                      C            k                    ·                                    (              Pe              )                        k                          ⁢                                            (                              1                -                Pe                            )                                      N              -              k                                .                    
In this case, the number of error bits counted by the parity check bit calculation is erroneously incremented by 1 when an even number of error bits occurs. Therefore, an error rate Pe—fec—bip obtained from the parity check bit after the error correction is calculated by the following expression:
      P          e_fec      ⁢      _bip        =                    (                  1          /          N                )            ·                        ∑                      k            =            1                                N            /            2                          ⁢                                  ⁢                  C                      2            ⁢            k                                          N                                  -                            (          Pe          )                          2          ⁢          k                    ⁢                                    (                          1              -              Pe                        )                                N            -                          2              ⁢              k                                      .            
Accordingly, there occurs an error between the error rate Pe—fec obtained after the error correction and the error rate Pe—fec—bip obtained from the parity check bit after the error correction. If Pe=10−7 and N=100, then Pe—fec=1.48×10−12 and Pe—fec—bip=4.9×10−13.
As described above, the conventional performance monitoring technique based on the existing parity calculation cannot provide a precise error rate.